Temperature control of solid-fueled grills is generally effected through manual throttling of a vent damper located in series with at least one of a combustion chamber inlet air vent, or combustion chamber exhaust vent. For example, during operation of a solid-fueled grill, the temperature of the grill may be reduced by at least partially closing one or more of the combustion chamber inlet or exhaust vent dampers. This action reduces the flow of fresh air for combustion to the fuel, thus resulting in a reduction of the rate of heat energy released by the combustion process. Conversely, the operating temperature of a solid-fueled grill may be increased during operation by at least partially opening one or more of the combustion chamber inlet or exhaust vent dampers.
Many charcoal or wood burning grill operators cook foods for long durations; in many cases twelve or more hours at low temperatures. It is problematic for the grill operator to have to frequently inspect the grill operating temperature and adjust the combustion chamber vent damper settings to maintain a target temperature for such long duration cooking cycles. Furthermore, this type of manual temperature control does not result in consistent and accurate operating temperatures, but rather variable and inconsistent operating temperatures.
Numerous factors such as variable fuel characteristics, ambient weather (temperature, humidity, precipitation, etc.) and drippings from the food item falling into the combustion chamber affect the temperature response of the grill to a given damper setting. Therefore, it is not possible to determine a correct damper setting for a given target cooking temperature prior to beginning a cooking cycle. Furthermore, during multi-hour cooking operations, ambient conditions and fuel characteristics are likely to vary, resulting in a continuously changing “correct” damper setting required to maintain a given target temperature in the cooking region.
Several apparatuses and methods for temperature control of solid-fueled grills appear in the prior art that employ electrically-driven blowers controlled by digital microcontrollers such that the blower controls the flow rate of fresh air through the combustion chamber, and is intermittently cycled on/off at varying duty cycles in accordance with a user-specified target temperature. U.S. Pat. Nos. 7,263,990 and 7,516,692, and US Patent Application Publication 2003/0015188 A1 are examples of such devices with electrically-driven blowers.
A major functional problem with these and any forced air temperature control is that blower systems are only effective at increasing airflow through a combustion chamber, and cannot provide significant resistance to naturally occurring airflow due to the “chimney effect” when powered off.
The chimney effect is naturally occurring flow of air established as hot air exits the combustion chamber through an exhaust vent that is positioned at a higher elevation than an inlet vent. The natural force that drives the hot exhaust from the combustion chamber is the action of buoyancy. Continuity results in fresh air being drawn into the inlet vent to replace a portion of the exiting exhaust gas. When a forced air temperature controller is deactivated, combustion airflow rate is established by the chimney effect, and the passage area of the blower flow path and/or the passage area of dampers in series with the combustion airflow path. Activation of a blower (as in the prior art mentioned above) can increase airflow, but it cannot reduce it.
Because of this chimney effect, forced air temperature control apparatuses are not well suited to cooking operations that require substantial changes in operating temperature during the cooking cycle. For example, a grill operator employing a forced air device might adjust combustion chamber vent dampers that are in series with the blower to yield a flow passage area small enough to require the blower to be energized to achieve the required airflow rate for proper operation. However, the operator must also determine that the chosen flow passage area is not so small that the blower is prevented from achieving the required airflow rate.
This characteristic of forced air control systems requires the operator to guess an appropriate damper setting prior to the cooking operation, and additionally requires the operator to monitor the progress of the cooking operation to ensure that the control device is achieving and maintaining the target temperature. Moreover, if the intended cooking cycle entails a variable temperature profile consisting of a period of operation at high temperature followed by or preceded by operation at a greatly reduced temperature, the operator may be obliged to reposition combustion chamber vent dampers in accordance with the changing requirements of the blower system.
An additional problem with forced air control systems is that they may not quickly recover from elevated combustion rates and temperature “flare ups” associated with the grill operator opening the lid of the grill to access the food, or from combustible drippings from food reaching the combustion chamber. Again, this is because they are only suited to increase airflow rates, not to resist them.
Yet another problem with these forced air control devices is that they require significant and continuous electrical current to operate, eliminating small battery packs or the like as a practical electrical source for extended duration operation, and requiring that a user have a permanent power source nearby (such as a wall outlet) or a large battery pack that will provide power for a sufficiently lengthy period.
Other attempts in the prior art to solve the problem include thermomechanically-actuated devices. U.S. Pat. No. 4,430,985, entitled “Thermostatically Controlled Charcoal Cooker” suggests the use of a thermo-mechanical system that actuates inlet and exit airflow valves to control fresh airflow rate through the combustion chamber, thus controlling fuel burn rate and internal temperature. However, this patent describes an open-loop proportional control system, which can only be accurate at the exact condition for which it was designed, and cannot adapt to varying airflow requirements for a given temperature when subjected to off-design conditions or disturbances. Essentially, this system could only perform with “ballpark” accuracy.
U.S. Pat. No. 4,434,781, entitled “Thermally efficient barbecue grill” details a positive feedback proportional control system. In this device, dampers open in direct proportion to heat generation. This arrangement is not practical for a solid fuel (charcoal) burning grill and could only be stable for a gas grill configuration, as the heat output would not increase with dampers opening without a commanded increase in gas fuel flow rate.
U.S. Pat. No. 6,223,737, entitled “Pellet fuel burning device” describes a grill that burns special wood or charcoal pellets, and controls operating temperature according to a user-specified target. Control is affected through the use of an electrically-driven auger that varies the rate of fuel delivery from a storage hopper to the combustion chamber. In addition, an electrically-driven blower is used to control airflow to the combustion chamber. This type of temperature control scheme suffers from all of the drawbacks of the above mentioned blower systems, and is also disadvantageous in that it requires many moving parts (subject to wear and tear), consumes a large amount of electricity, is complicated to operate, and requires specially-fabricated fuel pellets (as opposed to widely-available wood or charcoal pieces).
Accordingly, there persists a need for accurately controlling the cooking temperature for solid-fueled grills and ovens which overcomes all of the disadvantages of the prior art. There also persists a need for an apparatus for adjusting cooking temperature as required by the cooking process and for maintaining a desired temperature as the conditions in and around the grill or oven change. Furthermore, there exists a need for an apparatus that does not require a great deal of power to operate, and is easy to use and to manufacture.